The present invention relates to filters for the removal of particulate material from diesel engine exhaust streams, and more particularly to porous ceramic diesel exhaust filters of improved resistance to melting and thermal shock damage under conditions encountered in diesel exhaust systems.
U.S. Pat. No. 4,420,316 to Frost et al. discusses conventional ceramic wall-flow filter designs suitable for particulate filtration from diesel exhaust gases. As noted in that patent, cordierite ceramic materials of the kinds disclosed in U.S. Pat. Nos. 3,885,977 and 4,001,028 are commonly used for diesel particulate trap filters because they have properties which enable them to withstand the chemical and physical conditions to which they are subjected in the exhaust gas conditions found in diesel engines.
For diesel particulate filtration with these materials, honeycomb structures having cellular densities between about 10 and 300 cells/in2 (about 1.5 to 46.5 cells/cm.sup.2), more typically between about 100 and 200 cells/in.sup.2 (about 15.5 to 31 cells/cm.sup.2), are considered useful to provide sufficient thin wall surface area in a compact structure. Wall thicknesses can vary upwards from the minimum dimension providing structural integrity, about 0.002 in. (about 0.05 mm.), but are generally less than about 0.06 in. (1.5 mm.) to minimize filter volume. A range of between about 0.010 and 0.030 inches (about 0.25 and 0.76 mm.) e.g., 0.017 inches, is most often selected for these materials at the preferred cellular densities. Filter aspect ratios (the ratio of filter length to filter diameter) in the range of 0.8-1.2 are usually specified to provide adequate filtration volume in a packaged unit of manageable diameter.
Interconnected open porosity of the thin walls may vary, but is most generally greater than about 25% of thin wall volume and usually greater than about 35% to allow fluid flow through the thin wall longer dimensions. Diesel filter integrity becomes questionable above about 70% open pore volume; volumes of about 50% are therefore typical. For diesel particulate filtration it is believed that the open porosity may be provided by pores in the channel walls having mean diameters in the range of about 1 to 60 microns, with a preferred range between about 10 and 50 microns.
Filtration efficiencies up to and in excess of 90% of the diesel exhaust particulates (by weight) can be achieved with the described cordierite materials. The filtration of a lesser but still significant portion (i.e. less than 50%) of the particulates may be desirable for other filtering applications including exhaust filtering of smaller diesel engines. Efficiencies, of course, will vary with the range and distribution of the size of the particulates carried within the exhaust stream. Volumetric porosity and mean pore size are typically specified as determined by conventional mercury-intrusion porosimetry.
A significant problem with these conventional filter designs, however, is susceptibility to damage during the required filter regeneration cycling. In conventional diesel exhaust filtration systems, exhaust gas flow rate and thus engine operating efficiency are affected by the increasing pressure drop across the filter caused by the accumulation on the filter of collected carbonaceous exhaust particulates (soot). Eventually, this pressure drop becomes unacceptable and regeneration of the filter becomes necessary. In conventional systems, the regeneration process involves heating the filter to initiate combustion of the soot, a process that is highly exothermic and, if uncontrolled, produces temperature spikes that can thermally shock and crack, or even melt, the filter structure.
This problem has been recognized and several approaches to address it have been proposed. One such approach involves the careful selection of filter design parameters, as reported by H. Mizuno, J. Kitagawa, and T. Hijikata in "Effect Of Cell Structure On Regeneration Failure Of Ceramic Honeycomb Diesel Particulate Filter", SAE Paper No. 870010 (1987). Within a filter cell density range of about 100-200 cells/in.sup.2, Mizuno et al. found that increasing filter bulk density through increases in filter wall thickness and cell density effectively increased filter heat capacity, thereby moderating the peak filter temperatures encountered during filter regeneration. However, the thicker-walled filters exhibited higher pressure drops than standard filters, and in addition appeared to reach higher exhaust back-pressure levels in shorter times and at lower soot loadings than filters with thinner walls. These disadvantages suggested that an optimal combination of soot capacity and regeneration performance resided in a relatively narrow range of filter geometry characterized by filter wall thicknesses in the 0.012-0.017 inch range and cell densities in the 100-200 cells/in2 range.
Notwithstanding these findings, cordierite diesel exhaust filters produced in accordance with these guidelines, including standard commercial filters with cell densities of 100 cells/in and wall thicknesses of 0.017 inches, continue to experience thermal failures with unacceptable frequency in use and under test conditions approximating those found in diesel exhaust systems. Typical failures include filter cracking resulting from thermal stress and shock, as well as occasional melting resulting when large soot deposits undergo rapid and uncontrolled combustion.